The present invention relates to a high-performance electrical double layer capacitor formed by utilizing the principle of electrical double layer.
Electrical double layer capacitors are conventionally used as a backup power supply for semiconductor memory, as an auxiliary power source for electronic devices (such as microcomputers and IC memories), as a battery for solar watches, and as a power source to drive motors. They have recently been expected to find use as a power source for electric vehicles and as an auxiliary unit for energy conversion and storage systems.
In the case of an electrical double layer capacitor with electrodes formed from carbonaceous material, such as activated carbon, activated carbon fiber, and carbon black, its electric capacity depends basically on the surface characteristics and the BET specific surface area (measured by nitrogen adsorption) of the carbonaceous material, and its internal resistance is governed by the electric conductivity of the electrolyte. Therefore, there has been an increasing trend toward employing an electrolyte with a high electrical conductivity for reduction in internal resistance and for improvement in quick recharging performance and heavy current discharging performance.
Electrical double layer capacitors vary in appliable voltage depending on the kind of electrolyte employed therein. The appliable voltage is 2V or less in the case of an aqueous electrolyte formed by dissolving a supporting salt in water. By contrast, it is about 3V in the case of a non-aqueous electrolyte. In practical use, however, when a voltage of 2.5V or more is applied, the advantage of a non-aqueous electrolyte over an aqueous one diminishes because of its drawbacks, such as decomposition evolving gas and side reactions to dissolve the casing, leading to liquid leakage and capacity decrease. In addition, a non-aqueous electrolyte has a disadvantage of wasting electric power due to the leak current which flows when charging voltage remains on after full charging has occurred.
The present inventors have found that the electrical double layer capacitor greatly increases in withstanding voltage if the amount of supporting salt is reduced to such an extent that the electrolyte substantially becomes an insulator after the electrical double layer has been formed. They employed a process which consists of moving close to the electrode surface almost all the ionized supporting salt contained in the electrolyte when fully charged. This process creates a state in which, even when a high voltage remains applied, the electrolyte remote from the electrode is free of the ionized supporting salt which brings about side reactions. As a result, the electrolyte becomes nearly an insulator after the electrical double layer has been formed by full charging. Thus, a higher voltage can be applied without side reactions, and leak current after full charging decreases, resulting in power saving. In addition, the present inventors have found that the withstanding voltage increases with the decreasing ratio of the negative electrode capacity to the positive electrode capacity.